Distribution Network With LVRT Capable PV Power Supply Relay Protection Research

In the 21 st century,photovoltaic grid-connected power generation technologyhas gradually matured,and has been applied more and more widely in agriculture andindustry and commerce..With the continuous development of society,the number of photovoltaic power sources will continue to grow.However,with the incorporation of moreand more photovoltaic power into the power system,the reliability and security of thepower grid are affected.The traditional relay protection scheme of distributionnetwork is not applicable to the power grid with photovoltaic system,and theprotection may cause misoperation or refusal of operation.In this paper,the relay protection of PV distribution network with LVRT capability is studied in depth,and an adaptive current protection scheme is proposed,which improves protection range of the traditional distribution network protection device.The main research contents of this paper include:Firstly,Aiming at the structure of the two-level three-phase photovoltaic grid-connected power generation system,this paper establishes the mathematical model of photovoltaic inverters,designs the MPPT control algorithm,and completes the maximum power point tracking control.Based on the rotating coordinate system of the grid voltage vector orientation,this paper designs the double closed-loop controlstrategy of the photovoltaic inverter.Secondly,in view of the dq-PLL phase-locked loop cannot detect the fault voltage asymmetry situation,design a kind of based on SOGI-PLL phase-locked loop voltage sag detection method,according to the photovoltaic(pv)grid power generation system both at home and abroad LVRT(low voltage across)technical requirements,to solve the problem of voltage sags,proposed asymmetric fault voltage vector oriented control strategy,And positive and negative sequence current superposition vector control strategy to suppress asymmetric faults to provide a certain amount of reactive power support to complete LVRT control of inverter under grid faults.Then,according to the LVRT control strategy of the photovoltaic grid-connected system,a short-circuit calculation model for photovoltaic systems connected to the10 KV distribution network is established,and the traditional short-circuit current calculation method of the grid and the short-circuit current calculation method including the iterative correction of the photovoltaic system distribution network fault are introduced.According to the fault current characteristics of photovoltaic grid-connected inverters with LVRT capability,three-phase short-circuit simulations at the end of the distribution network without photovoltaic power sources,and three-phase short-circuit simulations at different locations in the distribution network with photovoltaic power sources were set up.The traditional A comparative analysis of the short-circuit current when a voltage drop occurs in the distribution network and the short-circuit current of photovoltaic power sources with LVRT capability into the distribution network.Finally,on the basis of analyzing the protection principle of traditional distribution network,the influence of different fault point positions of photovoltaic system integrated into distribution network on grid protection is studied.Based on the amount of local information,a suitable The adaptive current improvement protection scheme of photovoltaic power station distribution network with LVRT capability is simulated in Matlab software.The results show that the proposed adaptive protection setting principle is feasible.Compared with the traditional current break-off protection,the sensitivity and protection range are improved.